1. Field of the Invention
The present invention relates to seat reclining devices, and more particularly to seat reclining devices of a type which uses a planetary gear mechanism as an essential part of the device.
2. Description of the Prior art
Hitherto, for providing seat occupants with a comfortable sitting posture, various seat reclining devices have been proposed and put into practical use particularly in the field of passenger motor vehicles.
Some of them are of a type which uses a planetary gear mechanism as an essential part of the device, which are shown in for example Japanese Utility Model First Provisional Publications Nos. 63-85138 and 63-95853. Due to the nature of the planetary gear mechanism, a smoothed stepless pivoting of the seatback relative to the seat base is obtained.
However, even in the seat reclining devices of such type, prolonged usage tends to produce undesirable play of parts of the reclining device due to inevitable abrasion of them, which play makes the stepless pivoting of the seatback stiff and poor. Furthermore, when, due to a vehicle collision or the like, an abnormally big force is applied to the device, cancellation of locked condition of the gear parts tends to occur, which causes undesired suddent pivoting of the seatback relative to the seat base.